Vehicle fuel systems include evaporative emission control systems designed to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system may allow the vapors to be purged into an engine intake manifold for use as fuel.
The purging of fuel vapors from the fuel vapor canister may involve opening a canister purge valve coupled to a conduit between the fuel vapor canister and the intake manifold. During a purge operation, vacuum or negative pressure in the intake manifold may draw air through the fuel vapor canister enabling desorption of fuel vapors from the canister. These desorbed fuel vapors may flow through the canister purge valve into the intake manifold. As such, the canister purge valve may regulate the flow of fuel vapors into the intake manifold via a sonic choke positioned between a valve in the canister purge valve and the intake manifold. Accordingly, the sonic choke may function as a flow restrictor in the purge path between the valve and the intake manifold.
In boosted engines, during boost conditions when the compressor is operative, the intake manifold may have a positive pressure. Herein, an aspirator coupled in a compressor bypass passage may generate vacuum that can be used to draw stored fuel vapors from the fuel vapor canister. However, purge flow through the aspirator may be lower because the sonic choke in the canister purge valve may excessively restrict canister purge flow to the suction port of the aspirator. Accordingly, a performance of the aspirator in terms of purging the fuel vapor canister may be severely diminished by the presence of the sonic choke in the flow path.
An example approach demonstrating an improved purging operation is shown by Stephani in DE 102011084539. Herein, an aspirator coupled in the compressor bypass passage directly communicates with the fuel vapor canister such that fuel vapors are purged to the aspirator from the fuel vapor canister without flowing through a canister purge valve. By directly coupling the fuel vapor canister to the aspirator, the metering effect of the sonic choke in the canister purge valve may be circumvented. A diverter valve in the compressor bypass passage regulates flow through the aspirator and therefore, purging of the fuel vapor canister.
The inventors herein have identified potential issues with the above approach. As an example, transient engine conditions may be adversely affected by purging of the canister and compressor bypass flow. This can have negative consequences, including loss of engine power and efficiency, and an increase in combustion instability. Further, the approach in DE 102011084539 is primarily used during non-idle conditions when the aspirator can generate a vacuum to draw purged fuel vapors. Accordingly, manifold vacuum during idle conditions may not be availed for canister purging.
The inventors herein have recognized the above issues and identified an approach to at least partly address the issues. In one example approach, a method for a boosted engine comprises, during boosted conditions, flowing stored fuel vapors from a canister into an ejector, the flowing bypassing a canister purge valve and being regulated by a shut-off valve (SOV) positioned upstream of the ejector, and responsive to an operator tip-in event, closing the SOV, and discontinuing the flowing of stored fuel vapors from the canister into the ejector. Thus, by at least temporarily closing the SOV during transient engine conditions, improved engine performance can be achieved.
In another example approach, a method comprises, during boosted conditions, closing a canister purge valve (CPV), adjusting an opening of a shut-off valve (SOV) positioned upstream of an ejector in a compressor bypass passage, and flowing fuel vapors from a canister only to the ejector, the flowing regulated by the SOV and bypassing the CPV, and during non-boosted conditions, closing the SOV, opening the CPV, and flowing fuel vapors from the canister only to the CPV, the flowing bypassing the ejector. In this way, the canister may be purged during boosted as well as non-boosted conditions.
For example, a boosted engine may include an ejector positioned in a compressor bypass passage fluidically coupled to a fuel vapor canister. The boosted engine may also include a canister purge valve comprising a valve and a sonic choke. The valve may be a solenoid valve. Further, the sonic choke may be positioned downstream of, and proximate to, the valve in the canister purge valve within a single, common housing. An outlet of the sonic choke in the canister purge valve may be fluidically coupled to an intake manifold.
The fuel vapor canister may communicate with each of an inlet of the canister purge valve and a suction port of the ejector via distinct and separate passages. As such, stored fuel vapors from the fuel vapor canister may be purged directly to the ejector without flowing through a canister purge valve. Motive flow through the ejector may be controlled by a shut-off valve coupled to the compressor bypass passage. The shut-off valve may also regulate purge flow through the ejector by controlling the motive flow. During boosted conditions, the shut-off valve may be adjusted to a mostly open (or fully open) position and the ejector may generate vacuum due to the flow of compressed air in the compressor bypass passage. This ejector vacuum may draw stored vapors from the fuel vapor canister into an inlet of the compressor. Herein, fuel vapors may stream from the fuel vapor canister directly to the ejector while bypassing the canister purge valve. During non-boosted conditions, vacuum in the intake manifold may be applied to the canister purge valve and stored fuel vapors may be drawn from the fuel vapor canister into the intake manifold via the valve and the sonic choke bypassing the ejector. Thus, the fuel vapor canister may be purged during boosted and non-boosted engine conditions. In response to a tip-in event, the shut-off valve in the compressor bypass passage may be adjusted to a more closed position to enable a rapid rise in boost levels. Accordingly, purging from the fuel vapor canister may be temporarily discontinued during transient engine conditions as the ejector may not generate any vacuum when the shut-off valve is closed.
In this way, fuel vapors stored in a fuel vapor canister may be purged during boosted and non-boosted conditions in a turbocharged engine. By directly coupling the ejector to the fuel vapor canister, the sonic choke in the purge path via the canister purge valve may be circumvented and a purge flow rate to the compressor inlet may be enhanced. As such, the canister may be purged of its fuel vapors in the presence or absence of engine boost. Further, by controlling compressor bypass flow and ejector vacuum via the shut-off valve based on engine conditions, engine performance may be enhanced. Overall, vehicle fuel economy and emissions compliance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.